1. Field of Invention
This invention relates to instrumentation for measuring fluid motion, specifically the measurement of fluid motion at multiple points.
2. Discussion of Prior Art
Particle Image Velocimetry (PIV) is a technique in which one can measure the velocity of the flow at many, often thousands, of points in the flow simultaneously. Accurate velocity measurements of fluid motion using Particle Image Velocimetry (PIV) are typically on the order of 1 mm (see U.S. Pat. No. 5,333,044 to Shaffer, 1994, U.S. Pat. No. 1991000671529 to Komerath, 1991, U.S. Pat. No. 5,708,495 to Pitz, 1998, U.S. Pat. No. 5,979,245 to Hirano et al. 1999, Northrup, et al., 1991, and a review by Adrian, 1991).
U.S. Pat. No. 5,333,044 to Shaffer, 1994, claims a method for using fluorescent particle images to measure velocity of a flowing fluid close to solid surfaces. According to Shaffer, 1994, an optical apparatus is used to deliver a sheet of light to excite fluorescently-dyed particles in a flowing fluid (see FIGS. 2 & 3 of Shaffer, 1994). The two-dimensional measurement plane is defined by a sheet of illuminating laser light, following common practice in the art of Particle Image Velocimetry. Furthermore, in the invention of Shaffer (1994), the sheet of illuminating light is delivered by an optical apparatus separate from the optical apparatus that receives the fluorescent light and images the fluorescent light onto the image recording device (i.e. the CCD camera). In most practical applications of micron-resolution PIV, optical access is very limited, and it most practical applications of micron-resolution PIV, optical access is very limited, and it is necessary to use the same optical apparatus both to deliver the illumination light and to receive the filtered fluorescent light from the particles. This is not possible with the invention described by Shaffer (1994).
The PIV technique was extended by Urushihara, et al. (1993) and then by Keane et al. (1995) to obtain velocity measurements with spatial resolutions on the order of 100–200 microns.
The first attempt at micron resolution velocimetry was conducted by Brody et al. (1996). They estimated velocity by measuring the image streaks of 0.7 micron diameter particles through a microscope. The resulting velocity measurements were sparse, randomly spaced, low quality, and only accurate to within about 30% full scale. In addition this technique was limited to relatively low velocities.
Lanzillotto et al. (1996) used an X-ray micron-imaging technique to image 1–20 micron diameter emulsion droplets flowing in water. The technique requires a synchrotron to generate the X-rays. We estimate the spatial resolution of this technique to be about 40–100 microns. The accuracy of the technique is limited because of noise in the image field, the size of the emulsion droplets (1–20 microns), and the dispersion of the emulsion droplets relative to the working fluid.
Paul et al. (1997) used a technique related to PIV to analyze to motion of fluorescent dye. We approximate the spatial resolution of this experiment to be on the order of 100 μm×20 μm×20 μm, based on the displacement of the fluorescent dye between exposures, and the thickness of the light sheet used to uncage the fluorescent dye. This technique can be used to measure only one component of velocity with reasonable accuracy.
Hitt, Lowe & Newcomer (1996) used a technique related to PIV, known at Optical Flow, to measure in vivo blood flow in microvascular networks. They used seed particles with diameters on the order of 10 microns. Their measurements were noisy and have low accuracy. We estimate the spatial resolution of this technique to be at best 20 microns in each dimension.
Laser Doppler Velocimetry (LDV) has been a standard technique in fluid mechanics more than 25 years. However, LDV systems can only measure velocities at single points. The spatial resolution of LDV systems is usually on the order of a few millimeters. However, there have been several attempts to increase the spatial resolution to a few microns. Compton & Eaton (1996) used short focal length optics to obtain measurements with spatial resolutions of 35 microns×66 microns. Tieu, Machenzie, & Li (1995) built a dual-beam solid-state LDA system that had a measurement volume of approximately 5 μm×10 μm. Gharib, Modares & Taugwalder (1998) have developed a Miniature Laser Doppler Anemometer (MLDA), which can be designed to have a measurement diameter (spatial resolution) as small as 10 microns. These LDV systems are limited because they all measure velocity at only a single point.
The Optical Doppler Tomography (ODT) system developed by Chen et al. (1997) uses 1.7 micron diameter particles to measure velocity with a lateral and longitudinal spatial resolution of 5 microns and 15 microns, respectively. The system is noisy and is limited (like LDV) to pointwise measurements.Objects and Advantages
Accordingly, several objects and advantages of the current invention are:                (a) to measure flow velocity with higher spatial resolution than other Particle Image Velocimetry (PIV) techniques;        (b) to measure flow velocity at many (often hundreds to thousands) points simultaneously throughout the flow field;        (c) to measure flow velocity at regularly spaced grid points simultaneously throughout the flow field;        (d) to measure flow velocity with low noise and high accuracy;        (e) to measure flow velocity accurately very close to surfaces;        (f) to measure flow velocity over a large range of magnitudes;        (g) to measure flow velocity with high temporal resolution.        
Additional objects and advantages are:                (a) to measure instantaneous structures in the flow field, including but not limited to air bubbles and meniscus shapes and positions in liquid flows;        (b) to measure high resolution velocity fields without using fluorescent particles;        (c) the ability to measure flow inside non-transparent devices;        
Further objects and advantages will become apparent from a consideration of the drawings and ensuing description.